1. Field of the Invention
The present invention relates to a semi-passive type fuel cell system, and more particularly, to a semi-passive type fuel cell system having air uniformly supplied to air paths formed in a bipolar plate of a fuel cell stack.
2. Description of the Prior Art
As is generally known in the art, a fuel cell is an electricity generation system which directly converts chemical reaction energy generated between hydrogen contained in a hydrocarbon-based fuel such as methanol, ethanol and natural gas and an oxidant, into electrical energy.
Fuel cell systems are largely classified into a polymer electrolyte membrane fuel cell (hereinafter, referred to as a “PEMFC”) system and a direct methanol fuel cell (hereinafter, referred to as a “DMFC”) system.
In general, the PEMFC system includes a stack for generating electrical energy Through reaction between hydrogen and oxygen and a reformer for generating hydrogen through reforming a fuel. While the PEMFC system has great energy density and high output, hydrogen gas must be carefully handled, and the PEMFC requires an additional facility such as a reformer for reforming the fuel such as methanol, ethanol and natural gas and thereby producing hydrogen as fuel gas.
In the DMFC system, methanol as a fuel and oxygen as an oxidant are directly supplied to a stack to produce electricity through electrochemical reaction. The DMFC system has high energy density and high power density. Also, because a liquid fuel such as methanol is directly used, an additional facility such as a reformer is not required, and it is easy to store and supply the fuel.
In the DMFC system, air is forcibly supplied to the stack using air supply means such as an air compressor, an air pump, or the like. Because the DMFC system can be carried about, applicability to a portable terminal such as a notebook computer, a mobile phone, and the like, is being considered. However, the air supply means used in the DMFC system creates high noise levels and therefore is likely to cause inconvenience to a user. In consideration of this fact, the DMFC system has been developed as a passive type DMFC system in which air is supplied to the stack through natural convection or as a semi-passive type DMFC system in which air is supplied to the stack by virtue of blowing means such as a blower.
In the passive type DMFC system, because air is supplied through natural convection, air cannot be sufficiently supplied to respective cathodes. Hence, in the passive type DMFC system, unit cells are not stacked but arranged on a plane, whereby the area occupied by the stack is increased.
On the other hand, in the semi-passive type DMFC system, because the blowing means such as a blower is used, an amount of supplied air is increased when compared to the passive type DMFC system. By this fact, in the semi-passive type DMFC system, unit cells can be stacked to form a stack. In this regard, Japanese Unexamined Patent Gazette 2001-6717 discloses a fuel cell body in which a pair of electrodes including a fuel electrode and an oxidant electrode is located. Referring to FIG. 39, in order to replenish oxygen used in electrode reaction by the oxidant electrode of a fuel cell body 1, blowing means 5 for supplying oxidant gas is provided to an oxidant gas entrance, and the sectional area of an oxidant gas path that is formed in the oxidant electrode is gradually decreased from the oxidant gas entrance toward an oxidant gas exit.
Nevertheless, the semi-passive type DMFC system suffers from defects in that, although the blower is used, it is difficult to uniformly supply air to the air paths which are formed in the bipolar plate of the stack. Also, due to a difference in resistance to air flow, which is caused by the positions of the air paths, the amounts of air supplied to the respective air paths are rendered non-uniform.